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In-cylinder emission control strategies, such as modifications of injection pressure and injection timing, have been used by researchers in order to reduce exhaust emissions and to comply with the legislation standards. Since some years ago post-injections have been studied and are well known as being efficient for soot emissions reduction. Although is well known that diesel gaseous and particle mass emissions have been reduced progressively over the last twenty years in response to the restrictive emission legislation and due to the application of new technologies The aim of this work is to help develop and understand the effect of the post-injection on diesel exhaust particle size distributions. The approach is to use a modern, well instrumented research engine equipped with a flexible high pressure fuel injection system. The results of this work are available to help provide guidelines for strategies to achieve reductions of the particle size distributions in diesel engines. The major improvements with the post-injection in the particle size distributions was in the nucleation mode with the use of close post-injection with a high fuel mass post-injected; in the case of the accumulation mode certain relation was founded with the use of close post-injections with high quantities of fuel mass post-injected. In all cases the use of two post-injections strategy shows similar trends than the use of a single post-injection strategy.